The present invention relates to lasers, and in particular to lasers stabilized by selective wavelength feedback devices.
Many lightwave systems require laser sources that output a stable wavelength over a narrow-band or a broad-band for a wide range of operating conditions. For example, in certain telecommunication systems diode lasers are used to pump active elements (e.g., erbium-doped fiber amplifers) over a wide range of temperatures and drive currents for long periods of time. The requirements on the operational stability of diode lasers as a function of temperature and drive current, as well as diode laser aging have become more demanding as the performance requirements for the system as a whole have increased.
One approach to wavelength stabilization of a laser system is through frequency-selective feedback into the laser. By way of example, with reference to FIG. 1 there is shown a prior art two-cavity fiber Bragg grating stabilized diode laser system 10 comprising a diode laser 12 with rear and front facets 14 and 16, respectively. Front facet 16 has a reflectivity RFF. Adjacent rear facet 14 is a rear-facet monitor (RFM) 20 that is used to monitor output power and provide feedback to control the drive current supplied to the diode laser to maintain a constant output power.
An optical fiber 26 is optically coupled to diode laser 12 at front facet 16. A fiber Bragg grating 32 having a select reflectivity bandwidth (typically 1 nm or so) within the laser gain and a reflectivity RG is formed in optical fiber 26 a distance D1 from front facet 16.
In system 10, there are two discrete Fabry-Perot cavities. One cavity is within the diode laser itself between rear facet 14 and front facet 16, while the second cavity is an xe2x80x9cexternal cavityxe2x80x9d formed between the front facet and the fiber Bragg grating 32.
There are two main criteria for stable, broadband operation of system 10. The first is that the reflectivities RFF and RG need to be chosen such that the reflected light from the fiber Bragg grating is capable of causing system 10 to operate in what is known as the xe2x80x9ccoherence collapsexe2x80x9d regime. From the analysis of C. Henry and R. F. Karzarinov in their article xe2x80x9cInstability of semiconductor lasers due to optical feedback from distant reflectors,xe2x80x9d IEEE J. Quantum Electron., vol. QE-22, pp. 295-301, 1986, the relationship between the reflectivities RFF and RG for operation in the coherence collapse regime is:
RG greater than (0.01)RFF/(1xe2x88x92RFF)2
This relationship is satisfied over a very broad range of reflectivities, including the case where RGxcx9cRFF. For the case of coupling to an optical fiber, the coupling efficiency can be folded into an effective front facet reflectivity REFF 
The second criterion for stable operation of system 10 is that the distance D1 be greater than the coherence length L of the free running diode laser (i.e., in the absence of feedback from the fiber Bragg grating). This requirement ensures that the feedback light is incoherent with the light emitted from the diode laser. If the condition D1 greater than L is not met, diode laser 12 experiences instabilities at semi-periodic drive currents, which cause output power fluctuations (detected by RFM 20), which adversely impact the operation of the system (see, e.g., Achtenhagen et al, xe2x80x9cL-I characteristics of fiber Bragg grating stabilized 980-nm pump lasers, IEE Photonics Technology Letters, Vol. 13, No. 5, May 2001).
The coherence length L depends on the characteristics of the laser, and can be relatively large (i.e. greater than one meter). If the diode laser used is one that operates in a single longitudinal mode with a narrow bandwidth, the coherence length L can be relatively long, and substantially greater than that for a multi-longitudinal mode diode laser. A disadvantage of a large value of L is that the fiber Bragg grating must be placed very far away (e.g., meters) from the diode laser front facet in order to obtain stable operation of the system over the drive current operating range.
Increasing the distance D1 to ensure coherence collapse operation can create two problems. First, the required distance D1 can become impractical, particularly in applications where space is a concern such as pumped amplifier systems. Second, bi-modal behavior of the system can result from polarization rotation of the feedback light due to the natural birefringence in the fiber, which can be exacerbated by stressing or straining the fiber. This is particularly troublesome when D1 is large. For these reasons, it is preferred to keep the distance D1 as short as possible.
The problem of induced polarization rotation from fiber birefringence can be overcome by using a polarization-maintaining fiber. Polarization-maintaining fiber is designed to maintain the polarization axis of the guided light independent of stress and strain in the fiber, which can lead to polarization rotation. While polarization-maintaining fiber was developed for use in fiber-based optical systems where polarization needs to be maintained, in the present application the use of such fiber adds cost to the system and also makes the manufacturing process more complex because the fiber needs be precisely aligned to the diode laser during assembly.
A first aspect of the invention is a laser system that provides a stable output. The system includes a laser with front and rear ends surrounding a gain medium. A first wavelength-selective reflective element with a first reflectivity bandwidth is optically coupled to the laser. Optically coupled to the laser through the first wavelength-selective reflective element is a second wavelength-selective reflective element with a second reflectivity bandwidth that at least partially overlaps the first reflectivity bandwidth. The first and second wavelength-selective reflective elements are arranged such that the laser exhibits stable operation in the coherence collapse regime.
In a second aspect of the invention, an optical system is used in the laser system to couple the laser to the wavelength-selective reflective elements. The optical system may be arranged between the laser and the first wavelength-selective reflective element, may encompass the first wavelength-selective reflective element only, the second wavelength-selective reflective element only, encompass both wavelength-selective reflective elements, or be arranged between the wavelength-selective reflective elements. The optical system may comprise an optical fiber, and the wavelength-selective reflective elements may include fiber Bragg gratings or thin-film filters.
A third aspect of the invention is a method of forming a laser system that produces a stable output. The method includes providing a laser having an output end. First and second spaced apart wavelength-selective reflective elements having at least partially overlapping reflectivity bandwidths are provided adjacent the output end. The first and second wavelength-selective reflective elements are then coupled to the output end to provide substantially incoherent optical feedback to the laser. The laser may include a diode laser with front and rear facets. Further, the first and second wavelength-selective reflective elements may include, for example, thin-film filters arranged in free-space or fiber Bragg gratings arranged in an optical fiber as part of an optical system coupled to the laser.
A fourth aspect of the invention is a method of generating a stabilized laser output. The method includes providing a laser having rear and front ends surrounding a gain medium, and optically coupling the laser to first and second spaced apart wavelength-selective reflective elements. The elements have at least partially overlapping reflectivity bandwidths and are arranged to provide incoherent optical feedback to the laser such that the laser generates a stable laser light output in a coherence collapse regime.